Under certain operating conditions aircraft are vulnerable to accumulation of ice on components' surfaces. If unchecked such accumulations can eventually so laden the aircraft with additional weight and so alter the airfoil configuration of the wings as to cause an unflyable condition. Particularly vulnerable are low altitude, slow flying aircraft such as helicopters and tilt rotor aircraft. While a wide variety of systems have been proposed for removing ice from aircraft during flight or for preventing its accumulation on the leading edge surfaces of such aircraft, these prior art techniques can be placed into one of three general categories: thermal, chemical and mechanical.
Generally the first technique is known as thermal deicing. In one form the leading edges, that is, the edges of an aircraft component on which ice accretes and is impinged upon by the air flowing over the aircraft and having a point or line at which this air flow stagnates, are heated to loosen the accumulating ice from the aircraft. The loosened ice is blown from the aircraft component by the air stream passing over the aircraft. Heating is accomplished by placing a heating element in the leading edge zone of the aircraft component either by inclusion in a rubber boot or pad applied over the leading edge or by incorporation into the skin structure of the aircraft component. Electrical energy for the heating element is derived from a generating source driven by one or more of the aircraft engines. The electrical energy is switched on and off to provide heat sufficient to loosen accumulating ice. In another heating approach gases at elevated temperature from one or more compression stages of a turbine engine are conducted through passages and permitted to exit through the leading edges of components in order to prevent ice accumulation in the first instance or to heat accumulated ice to loosen the adhesive forces between it and the aircraft component. This latter system is often referred to as "bleed air" deicing. Both of these approaches require a considerable amount of power. The so-called bleed air systems result in reduced fuel economy and lower turbine engine power output available for thrust of the plane.
Generally the second approach is to apply a chemical to all or part of the aircraft to depress adhesion of ice to the aircraft or to depress the freezing point of water collecting upon surfaces of the aircraft.
The third commonly employed approach for deicing is generally termed mechanical deicing. The principal commercial mechanical deicing means employs a plurality of expandable generally tubelike structures which are inflatable employing a pressurized fluid, typically air. Upon inflation these tubular structures expand the leading edge profile of the wing or strut to crack ice accumulating thereon for dispersal into the air stream passing over the aircraft component. Exemplary of these structures are those described in U.S. Pat. Nos. 4,494,715 and 4,561,613 to Weisend, Jr. in which the pneumatic deicers are formed of compounds having rubbery or substantially elastic properties. Inflation of these tubes results in their expansion or stretching by 40% or more. The time for inflating such tubes typically averages between 2 and 6 seconds and results in a substantial change in profile of the deicer, as well as the leading edge, thereby cracking ice accumulating on the leading edge.
A more recently developed approach to pneumatic mechanical deicing is described in U.S. Pat. Nos. 4,706,911 to Briscoe et al. and 4,747,575 to Putt et al. Such deicers include a sheet-like skin having a substantially elevated modulus, a support surface positioned obversely with respect to the ice accreting surface, and one or more inflation tubes positioned between the support surface and skin. The inflation tubes are configured for inflation to an extent sufficient to deform the skin to a degree sufficient to dislodge ice accumulations upon the ice accreting surface without exceeding the stress endurance limit for the material from which the skin is formed.
Another subcategory of mechanical deicing includes those techniques that utilize internal "hammers" to distort the leading edge of the wing, as exemplified by U.S. Pat. No. 3,549,964 to Levin, wherein electrical pulses from a pulse generator are routed to a coil of a spark-gap pressure transducer adjacent the inner wall of the airfoil. The primary current in the coil induces a current in the wall of the airfoil and the magnetic fields produced by the currents interact so as to deform the airfoil wall. U.S. Pat. Nos. 3,672,610 and 3,779,488 to Levin; and 4,399,967 to Sandorff disclose aircraft deicers that utilize energized induction coils to vibrate or torque the skin on which ice forms. Each of these disclose electromagnetic coils or magneto restrictive vibrators located on the obverse surface of the skin on which ice accumulates. In U.S. Pat. No. 3,809,341 flat buses are arranged opposite one another with one side of each bus being adjacent an inner or obverse surface of an ice collecting wall. An electric current is passed through each bus and the resulting interacting magnetic fields force the buses apart and deform the ice collecting walls. The disadvantage of the aforedescribed electromechanical systems is that each operates on the structural skin of the airfoil and a predetermined skin deflection is required to provide a set level of ice removal. A large force is required in order to generate the needed amount of skin deflection. Such high skin deflections are believed likely to cause fatigue in the skin.
U.S. Pat. No. 4,690,353 to Haslim et al. describes another subcategory of electromechanical deicing. One or more overlapped flexible ribbon conductors embedded in an elastomeric material are affixed to the outer surface of an airfoil structure. The conductors are fed large current pulses from a power storage unit. The resulting interacting magnetic fields produce an electroexpulsive force which distends the elastomeric member and separates the elastomeric member from a solid body such as ice thereon. The distention is almost instantaneous when a current pulse reaches a conductor. In preferred embodiments having multiple electrical conductors, the electrical conductors each have a serpentine configuration.
The present invention is an improvement over that disclosed in U.S. Pat. No. 4,690,353. Applicants have found that the arrangement of the conductors, and particularly the direction of current flow in adjacent ones of the electrically conductive members can produce much greater electroexpulsive force than the serpentine configuration taught by Haslim et al. Applicants have found that delivery of a current pulse of predetermined magnitude, shape and duration provides more effective de-icing.